This invention relates to the recovery of fluids from subterranean formations penetrated by a well and more particularly to a process for stimulating the well and improving the production of fluids therefrom.
In the production of fluids, such as petroleum, from subterranean reservoirs by penetrating the reservoir with a well, the permeability of the formation adjacent the well bore is critical to the productivity of the well. In completing a well, particularly one from which petroleum is to be recovered, it is conventional to create a series of passages, referred to herein as perforation tunnels, which extend from the well bore into the formation so as to provide channels for directing the flow of fluids from the formation into the well bore. In a typical case the productivity of a well over a given period of time will drop off as a result of the decrease in permeability of the reservoir adjacent the well bore. Typically this can be accounted for by the accumulation in the formation pores of various materials, such as waxes, insoluble inorganic material, residue from drilling fluids and the like. In addition, productivity of the well is also reduced by the accumulation of these permeability reducing materials in the so-called "crushed zone" which surrounds the perforation tunnels. The "crushed zone" consists of compacted detritus produced during the formation of the tunnels.
Various methods are known in the art for stimulating the productivity of fluid producing wells by improving the permeability of the formation adjacent the well bore. For example, "acidizing" processes are utilized wherein an acid solution is injected into the well and caused to enter the pores of the formation to dissolve the acid soluble portion of the formation rock and permeability reducing materials accumulated in the formation pores adjacent the well thereby to enlarge and reopen the pores in the formation. Acidizing, however, has several disadvantages not the least of which is that it cannot be used where the formation is not susceptible to attack by acid. Also, unless inhibited, acid solutions are extremely corrosive to pumps, lines and the like and thus increase the cost of maintenance of the well. In addition, unless carefully controlled, acidizing can result in the extension of the well bore into undesirable levels of the formation by acid erosion, i.e. the extension of a petroleum producing well into the water zones.
Other means for stimulating a producing well include the injection of a solvent, hot solvent or steam into the well to dissolve or melt paraffin deposits, thus reopening the perforation tunnels and pores of the formation adjacent the well bore. However, solvent and steam techniques are effective only in those cases where loss of permeability is caused by the deposition of paraffins and waxes from crude petroleum and is not effective where loss of production is caused by inorganic deposits in the pores of the formation or drilling fluid deposits. Another alternative is to rework the well by plugging the existing perforation tunnels and forming new perforation tunnels or by fracturing the well by explosives or by hydraulic processes. These methods are time consuming and expensive and in the case of explosive fracturing it is necessary to replace a substantial portion of the well casing and the reinforcing cement work resulting in lost production time and substantial cost.